1. Field of the Invention
The present invention relates to an optical module for use in an optical communication system, and more particularly to an optical module including an optical isolator.
2. Description of the Related Art
An optical communication system generally includes optical transmitter and receiver modules for transmitting and receiving optical signals. In addition, the system includes an optical fiber, serving as an optical-signal transmission medium, for providing a connection between the optical transmitter and receiver modules, and various other optical parts such as an optical connector and/or an optical splitter. These optical parts are connected to each other to implement various functions of the optical communication system.
Backward reflection light causes disturbances and malfunctions in the system, particularly in the semiconductor lasers. Light that is partially reflected or scattered and travels backward, opposite to its original direction of travel, is caused by a number of situations—for example, if the different optical parts are not coupled properly or the medium for carrying optical signals is non-uniform.
A number of solutions have been proposed to prevent the disturbance and malfunction due to the backward reflection light of a light source such as the semiconductor laser. A widely used manner is an optical communication system, which employs an optical part such as an isolator, for confining the traveling direction of light to only one direction.
FIG. 1 shows an optical module including a conventional isolator and a semiconductor laser. As shown in this drawing, the optical module includes a semiconductor laser 110 for outputting light of a predetermined wavelength, and an isolator 120 for transmitting the light outputted from the laser 110 and blocking a backward reflection beam from being inputted to the laser 110.
The isolator 120 includes a polarizer 121, a Faraday rotator 122, and an analyzer 123. The polarizer 121 transmits light only of a predetermined linearly-polarized component. The rotator 122 rotates the linear polarization direction of light by 45°. The analyzer 123 transmits light only of a predetermined linearly-polarized component. It is assumed that the traveling direction of light outputted from the laser 110 corresponds to the Z-axis. The two orthogonal axes perpendicular to the Z-axis are the X and Y-axes.
The polarization axis of the polarizer 121 is parallel to the Y-axis. The polarizer 121 is disposed to face an end of the semiconductor laser 110. It transmits only a light beam coinciding with its polarization axis, among light beams outputted from the laser 110, to the Faraday rotator 122.
The Faraday rotator 122 is disposed so that its one end faces the polarizer 121. It rotates the linear polarization direction of light, which enters it through the polarizer 121, by 45° with respect to the polarization axis of the polarizer 121. Thereafter the light is output to the analyzer 123.
The analyzer 123 is disposed symmetrically to the polarizer 121 with the rotator 122 being positioned between the analyzer 123 and the polarizer 122. The analyzer's 123 polarization axis is inclined at 45° with respect to that of the polarizer 123. In other words, the polarization axis of the analyzer 123 is tilted at 45° to each of the X and Y-axes. The polarization axis of the analyzer 123 thus coincides with that of light outputted from the Faraday rotator 122. The light is linearly polarized and rotated by 45° so that the analyzer 123 transmits the light outputted from the rotator 122.
The isolator 120 blocks reflection light. In particular, the analyzer 123 transmits only a reflection beam linearly polarized at 45°, from the external reflection beams incident on the analyzer 123, to the Faraday rotator 122. The rotator 122 rotates the linear polarization direction of the reflection light, from the analyzer 123, by 45°. The light is thus perpendicular to the polarization axis of the polarizer 121. Then it is output to the polarizer 121. The polarizer 121 blocks the reflection light and is prevented from entering the semiconductor laser 110.
However, a conventional isolator requires an accurate optical-axis alignment so that the polarization axes of the polarizer and analyzer are correctly positioned at 45° to each other. The conventional isolator has a number of limitations including increased fabrication delay time due to such an optical-axis alignment. Further, the use of a large number of optical parts increases production costs.